1. Field of the Invention
The present invention is in the field of radio and more specifically is concerned with a circuit to produce simultaneous amplitude modulation of any number of carrier signals without causing intermodulation distortion or spurious signals that would adversely affect the quality of the modulated signals.
Such a circuit can be useful where it is desired to transmit an emergency message in an area such as a tunnel or a remote canyon where AM broadcast band reception may be blocked. Typically, vehicle radios entering such an area may be tuned to any of several known carrier frequencies.
In some locations, provision has been made for re-transmitting a number of AM channels into the area where normal broadcast reception is blocked, by use of a local transmitter.
Since the vehicle radios are already pre-tuned to several known carrier frequencies, it becomes necessary to apply the emergency audio signal simultaneously to each of the carriers used in re-transmitting the AM channels.
2. The Prior Art
The conventional prior art approach to the problem of applying the emergency audio signal simultaneously to each of the carriers would be to combine the carriers C.sub.1, C.sub.2, C.sub.3, . . . and then to modulate the combination, so that the broadcast signal B can be represented as EQU B=M(C.sub.1 +C.sub.2 +C.sub.3 + . . . )
where M is the modulating signal.
This conventional multiplicative approach leaves much to be desired. A plethora of unwanted signals are produced which cause interference and unintelligible reception in the broadcast receivers. This unfortunate result occurs because nonlinearities in the conventional AM modulator cause intermodulation distortion resulting in the generation of signals that are the sum and difference of any signal combinations within the circuit.
So far as I can determine, the problem of applying a single modulating signal to a number of carrier signals simultaneously has not been satisfactorily solved.